This disclosure relates to circuits for measuring the output current of a transformer and more particularly to circuits that provide an accurate measurement of the output current of a transformer that generates a high voltage, high frequency output signal that at least occasionally has a low output current.
In the process of electrophotographic printing, a charge-retentive surface, also known as a photoreceptor, is charged to a substantially uniform potential, so as to sensitize the surface of the photoreceptor. The charged portion of the photoconductive surface is exposed to a light image of an original document being reproduced, or else a scanned laser image created by the action of digital image data acting on a laser source. The scanning or exposing step records an electrostatic latent image on the photoreceptor corresponding to the informational areas in the document to be printed or copied. After the latent image is recorded on the photoreceptor, the latent image is developed by causing toner particles to adhere electrostatically to the charged areas forming the latent image. This developed image on the photoreceptor is subsequently transferred to a sheet on which the desired image is to be printed. Finally, the toner on the sheet is heated to permanently fuse the toner image to the sheet.
One familiar type of development of an electrostatic image is called “two-component development”. Two-component developer material largely comprises toner particles interspersed with carrier particles. The carrier particles are magnetically attractable, and the toner particles are caused to adhere triboelectrically to the carrier particles. This two-component developer can be conveyed, by means such as a “magnetic roll,” to the electrostatic latent image, where toner particles become detached from the carrier particles and adhere to the electrostatic latent image.
In magnetic roll development systems, the carrier particles with the triboelectrically adhered toner particles are transported by the magnetic rolls through a development zone. The development zone is the area between the outside surface of a magnetic roll and the photoreceptor surface on which a latent image has been formed. Because the carrier particles are attracted to the magnetic roll, some of the toner particles are interposed between a carrier particle and the latent image on the photoreceptor. These toner particles are attracted to the latent image and transfer from the carrier particles to the latent image. The carrier particles are removed from the development zone as they continue to follow the rotating surface of the magnetic roll. The carrier particles then fall from the magnetic roll and return to the developer supply where they attract more toner particles and are reused in the development process. The carrier particles fall from the magnetic roll under the effects of gravity or a magnetic field that repulses the carrier particles.
Different types of carrier particles have been used in efforts to improve the development of toner from two-component developer with magnetic roll development systems. One type of carrier particle is a very insulated carrier and development systems using developer having these carrier particles increase development efficiency through low magnetic field agitation in the development zone along with close spacing to the latent image and elongation of the development zone. The magnetic field agitation helps prevent electric field collapse caused by toner countercharge in the development zone.
The close spacing increases the effective electric field for a potential difference and the longer development zone provides more time for toner development. Other two-component developers have used permanently magnetized carrier particles because these carrier particles dissipate toner countercharge more quickly by enabling a very dynamic mixing region to form on the magnetic roll.
Another type of carrier particle used in two-component developers is the semiconductive carrier particle. Developers using this type of carrier particle are capable of being used in magnetic roll systems that produce toner bearing substrates at speeds of up to approximately 100 pages per minute (ppm). Developers having semiconductive carrier particles produce a relatively thin layer of developer on the magnetic roll in the development zone. Consequently, magnetic rolls used with semiconductive carrier particles rotate in the same direction as the photoreceptor. That is, rotation of the magnetic roll in the direction opposed to the rotation of the photoreceptor has been observed to be unable to supply an adequate amount of developer for solid halftones and other images.
Many known magnetic roll systems used with developers having semiconductive carrier particles use two magnetic rolls. The two rolls are placed close together with their centers aligned to form a line that is parallel to the photoreceptor. Because the developer layer for semiconductive carrier particle developer is so thin, magnetic fields sufficient to migrate semiconductive carrier particles in adequate quantities from one magnetic roll to the other magnetic roll also interfere with the transfer of toner from the carrier particles carried by the magnetic rolls.
Typically, the carrier and toner particles are freed from the magnetic rolls to form a toner cloud adjacent the photoreceptor. Pairs of wires are often placed in the region between the magnetic rolls and the photoreceptor so that magnetic fields can be generated to cause the toner and carrier particles to be released from the magnetic rolls. These wires are typically supplied by a high voltage power supply in order to generate the necessary magnetic fields. Monitoring of the current supplied by the power supply is often utilized to control the fields generated by the wires. Unfortunately, current circuits and methods of monitoring the output of a high voltage power supply are often ineffective or inaccurate when the high voltage power supply creates a signal with high frequency components and an occasional low current output.
In the prior art, as shown, for example, in FIGS. 7 and 8, the output current of a high voltage transformer 102 is measured by means of a sense resistor 110 in the transformer high voltage winding 108 at the ground side 112 of this winding 108. Using this method, it is assumed that the voltage on the sense resistor 110, referenced to ground 116 is proportional with the output current. For low frequency waveforms this circuitry and method of measuring current work well but if the waveform consists of higher frequencies and the output current is low, this method is not suitable anymore because of the transformer capacitance. The transformer 102 exhibits a parasitic capacitance having two components, a parasitic capacitance between the high voltage winding 108 and the low voltage winding 106 and a parasitic capacitance between the high voltage winding 108 and ground 116. As shown, for example, in FIG. 7, the two components of the parasitic capacitance of the transformer 102 can be modeled by a high voltage to low voltage parasitic capacitor CHV-LV 118 (shown in phantom lines) and a high voltage to ground parasitic capacitor CHV-GND 120 (shown in phantom lines). As shown in phantom lines in FIG. 8, the high voltage to low voltage parasitic capacitor CHV-LV 118 and the high voltage to ground parasitic capacitor CHV-GND 120 can be modeled with a single equivalent parasitic capacitor CEQU 122 coupled between the hot node 114 of the high voltage winding 108 of the transformer 102 and ground 116. A parasitic current Ipar 124 flows through the equivalent parasitic capacitor CEQU 122 to ground 116. This additional current, parasitic current Ipar 124, is also flowing through the sense resistor Rsense 110, but it is not a portion of the output current. As shown, for example in FIG. 8, the current flowing through the sense resistor Rsense 110 is a current 128 represented by the combination of the output current 126 and the parasitic current 124 Consequently, the measured current does not represent the output current IOUT 126 accurately. If the waveform and amplitude are constant one could compensate for the current leakage by subtraction of an offset, but if this is not the case other techniques have to be used.
The described combination of factors is applicable for the high voltage power supply 100 required in many semi-conductive magnetic brush (“SCMB”) printers. Thus the disclosed measurement circuit utilizes a simulation capacitor and a second sense resistor for measuring the current through the simulation capacitor. The simulation capacitor simulates the total equivalent parasitic capacitance and is connected directly with the hot side of the transformer's high voltage windings. The real output current can be obtained using this arrangement by subtracting the simulation capacitor current (measured with the second sense resistor) from the measured total current. In order to reduce additional transformer load because of the simulation capacitor, the capacitance can be scaled down. This can be corrected with scaling up the corresponding sense resistor value.
According to one aspect of the disclosure, a current measurement circuit for measuring the output of a power supply having a signal generator inputting a signal to an input winding of a transformer exhibiting a parasitic capacitance capable of being modeled by an equivalent parasitic capacitor coupled between a hot terminal of an output winding of the transformer and ground is provided. The current measurement circuit comprises a simulation capacitor, a second sense resistor, a first sense resistor and a differential amplifier. The simulation capacitor has a capacitance proportional to the parasitic capacitance of the transformer. The simulation capacitor has a first electrode coupled to the hot terminal of the output winding of the transformer and a second electrode coupled to a first node. The second sense resistor is coupled to the first node and to ground so that the current flowing through the simulation capacitor flows through the second sense resistor. The first sense resistor is coupled to a second node through which a current having a component representative of the output current of the power supply and a component representative of the parasitic current flows. The differential amplifier is coupled at an inverting input to the first node and at a non-inverting input to the second node. The differential amplifier supplies an output signal proportional to the output current of the power supply.
According to a second aspect of the disclosure, a printer apparatus includes a photoreceptor, a magnetic roll, an electrode, a current source and a current measurement circuit. The magnetic roll is configured to attract development material including toner to a development zone adjacent the photoreceptor. The electrode is positioned adjacent the development zone and configured to induce toner transported by the magnetic roll to be released in the development zone by generating a magnetic field induced by a current flowing through the electrode. The current source is coupled to the electrode and supplies a current thereto for generating the magnetic field. The current source has a signal generator inputting a signal to an input winding of a transformer exhibiting a parasitic capacitance capable of being modeled by an equivalent parasitic capacitor coupled between a hot terminal of an output winding of the transformer and ground. The current measurement circuit comprises a simulation capacitor, a first sense resistor, a second sense resistor and a differential amplifier. The simulation capacitor has a capacitance proportional to the parasitic capacitance of the transformer. The simulation capacitor has a first electrode coupled to the hot terminal of the output winding of the transformer and a second electrode coupled to a first node. The second sense resistor is coupled to the first node and to ground so that the current flowing through the simulation capacitor flows through the second sense resistor. The first sense resistor is coupled to a second node through which a current having a component representative of the output current of the current source and a component representative of the parasitic current flows. The differential amplifier is coupled at an inverting input to the first node and at a non-inverting input to the second node. The differential amplifier supplies an output signal proportional to the output current of the current source.
According to yet another aspect of the disclosure, a method of measuring the current output by a power supply having a transformer exhibiting a parasitic capacitance capable of being modeled by an equivalent parasitic capacitor coupled to a hot terminal of an output winding of the transformer and ground through which a parasitic current flows when the output of the transformer is a high voltage low current signal having high frequency components is provided. The method comprises measuring a current including a component proportional to the output current of the power supply and a component proportional to the parasitic current and subtracting the component proportional to the parasitic current from the measured current.
Additional features and advantages of the presently disclosed current measurement circuit for a transformer with high frequency output will become apparent to those skilled in the art upon consideration of the following detailed description of embodiments exemplifying the best mode of carrying out the disclosed method and apparatus as presently perceived.
Corresponding reference characters indicate corresponding parts throughout the several views. Like reference characters tend to indicate like parts throughout the several views.